Method for establishing a release criterion for restraining means

ABSTRACT

A clear distinction between deployment and nondeployment situations is made by subjecting the measured acceleration of a vehicle to low-pass filtration, forming a deployment threshold as a function of the filtered acceleration signal, analyzing an acceleration signal, as soon as it is measured, in a first time segment to determine characteristic features indicating a crash event in which the restraint device should not be deployed and reducing the cut-off frequency of the low-pass filter for a predetermined time segment if a nondeployment situation of this type is detected.

FIELD OF THE INVENTION

The present invention relates to a method for forming a deploymentcriterion for means of restraint in a vehicle, where the vehicleacceleration in at least one direction is measured, the measuredacceleration is integrated, the integrated acceleration is compared to athreshold that assumes a high value at low accelerations and a low valueat high accelerations, and an exceeding of the threshold by theintegrated acceleration is used as the deployment criterion.

BACKGROUND INFORMATION

A method of this type for forming a deployment criterion, for means ofrestraint is known from European Patent No. 458796. In the deploymentalgorithm described in this publication, the level of the thresholddepends on the measured acceleration. Selecting a suitable thresholdmakes it possible to distinguish between deployment and nondeploymentsituations. Indeed, the deployment of means of restraint (airbags orseatbelt tensioners) should be prevented when the vehicle is involved inonly a minor crash or, for example, when driving over railroad ties orthe edge of a curb, thus preventing any risk of injury to the vehicleoccupants. In the case of serious crashes, on the other hand, in whichthere is always a risk of injury to the vehicle occupants, nothingshould prevent the means of restraint from deploying. Because there isoften only a very narrow margin between crashes that require deploymentof the means of restraint and those in which such deployment should beprevented, it is entirely possible that the wrong decision is made as towhether or not the means of restraint should be deployed.

The object of the present invention is therefore to provide a methodforming a deployment criterion as reliably as possible that clearlydistinguishes between nondeployment and deployment situations.

SUMMARY OF THE INVENTION

The object mentioned above is achieved in that the measured accelerationis subjected to low-pass filtration; the deployment threshold is formedas a function of the filtered acceleration signal. An accelerationsignal is analyzed, as soon as it has been measured, in a first timesegment to determine characteristic features indicating a crash event inwhich the means of restraint should not be deployed; and the cut-offfrequency of the low-pass filter is reduced for a predetermined timesegment if a nondeployment situation of this type is detected. Analyzinga measured acceleration signal in the beginning to determine whetherfeatures of a nondeployment crash are present, and influencing thedeployment threshold accordingly, produces a much more accuratedeployment criterion, which clearly distinguishes between deployment andnondeployment situations.

If a nondeployment crash were erroneously detected, although a seriouscrash requiring unconditional deployment of the means of restraint didindeed occur, the crash signals would be only slightly influenced by thebrief reduction in the cut-off frequency of the low-pass filter.

It is expedient that the integration of the measured acceleration doesnot begin until the acceleration signal exceeds a predetermined signallevel. Thus, very slight vehicle accelerations lying below a noisethreshold are initially completely ignored when forming the deploymentcriterion.

The low-pass filter can preferably be an IIR or FIR filter whoseparameters are varied upon detecting a nondeployment situation so thatthe filter cut-off frequency is reduced.

To detect nondeployment situations, the measured acceleration signal isadvantageously integrated across multiple consecutive, short timewindows in the first time segment, the acceleration signal integrated inthis manner is then subjected to a threshold value decision, and anondeployment situation is determined if the integrated accelerationsignal initially drops below a lower threshold due to a vehicledeceleration and subsequently exceeds an upper threshold due to elasticvibrations in the vehicle body.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram illustrating the derivation of a deploymentcriterion according to an embodiment of the present invention.

FIGS. 2a and 2 b show a measured acceleration signal and the integral ofthis acceleration signal.

FIG. 3 shows the dependency of the deployment threshold on acceleration.

FIGS. 4a and 4 b show a measured acceleration signal and a variation inthe deployment threshold assigned to this signal.

FIG. 5 shows a flowchart for detecting a nondeployment crash accordingto an embodiment of the present invention.

FIGS. 6a and 6 b show a measured acceleration signal and its variationfollowing window integration.

FIGS. 7a and 7 b show a measured acceleration signal and its variationfollowing filtration.

DETAILED DESCRIPTION

As illustrated by the block diagram in FIG. 1, an acceleration ameasured in the vehicle, preferably an acceleration in the direction ofits longitudinal axis, is supplied to an integrator INT. FIG. 2a showsan example of a variation in a measured acceleration a. The integrationof acceleration a does not begin immediately upon the appearance of theacceleration signal, but rather only after acceleration signal a hasreached a certain signal level. For this purpose, a level range isdefined by two thresholds P1 and P2, and integration does not begin aslong as the acceleration signal remains within this level range, sincethis acceleration signal would be much too small to deploy restraints.As indicated in FIG. 2a, integration time tint begins as soon asacceleration signal a exceeds threshold P1. Integration continues untilacceleration signal a has returned to the level range between P1 and P2for a period tr. The result of integrating acceleration signal a is achange in speed Δv, as shown in FIG. 2b. Within integration time tint,output signal Δv of integrator INT increases from 0 to a final valueachieved at the end of integration time tint. Speed change signal Δvoutput by integrator INT is supplied to a threshold value decider SE,which compares signal Δv to a deployment threshold Δvs. The deploymentcriterion for restraints in the vehicle is established by speed changesignal Δv exceeding deployment threshold Δvs. If the signal exceeds thethreshold value, deployment signal z appears at the output of thresholdvalue decider SE.

The way in which deployment threshold Δvs is determined is explainedbelow. Deployment threshold Δvs is formed in a block SW. As shown inFIG. 3, deployment threshold Δvs has a variation dependent on vehicleacceleration. Indeed, deployment threshold Δvs has a high value at a lowacceleration a and a low value at a high acceleration a. This ensuresthat high accelerations always meet the criterion for deployment. Lowaccelerations, on the other hand, should never cause deploymentthreshold Δvs to be exceeded, which is also prevented by a highdeployment threshold Δvs at low accelerations a. The precise variationin deployment threshold Δvs is derived empirically from a large numberof crash trials and optimized accordingly. As shown in FIG. 3, the levelof deployment threshold Δvs is also dependent on integration time tint.As integration time tint of acceleration a increases, deploymentthreshold Δvs decreases to a lesser extent from a high value to a lowone along with the rise in acceleration a. The example illustrated inFIGS. 4a and 4 b shows how, in the case of a measured acceleration a(FIG. 4a), a deployment threshold Δvs assigned thereto (FIG. 4b) variesover time t. FIG. 4b demonstrates that deployment threshold Δvs assumesa high value at low accelerations a and a low value at highaccelerations a.

Block NAE in FIG. 1 analyzes acceleration signal a in a first timesegment, lasting approximately 10 to 15 ms, starting at the beginning ofacceleration signal a. During this first time segment, accelerationsignal a is analyzed to determine characteristic features indicatingthat a nondeployment crash has occurred. FIG. 5 shows a flowchart thatillustrates the procedure for analyzing acceleration signal a. In afirst step 1, measured acceleration signal a is subjected to a windowintegration. This means that measured acceleration signal a is dividedinto multiple consecutive time segments and integrated in each timesegment. In practice, a time window lasting approximately 1.5 ms isformed for this purpose and is successively shifted over the first timesegment mentioned above, with the measured values of acceleration adetermined by the time window being integrated in each new time windowposition. The example shown in FIGS. 6a and 6 b illustrates thisprocedure. FIG. 6a shows an example of a variation in an accelerationsignal a. The values plotted on the ordinate axis refer to multiples ofground acceleration g. FIG. 6b shows acceleration signal Δa followingthe above-described window integration of measured acceleration signala. Due to this window integration, striking signal levels of measuredacceleration signal a become much more prominent. In the case ofnondeployment crashes, a sharper deceleration occurs at the beginning ofthe acceleration signal, followed by a number of higher accelerationlevels, due to elastic vibrations in the vehicle body. Thesecharacteristic nondeployment crashes include slow crashes occurring at avehicle speed of <20 km/h, or driving into gravel heaps or driving overcurbs or crossing railway ties, and the like. Slow crashes at a vehiclespeed below 20 km/h are often elastically absorbed by the front-endstructure of the vehicle body without any significant breaks forming.One or more pronounced deceleration peaks thus occur in the beginning,followed immediately by acceleration components that reflect the elasticbehavior of the vehicle body.

The described signal levels of window-integrated acceleration signal haare detected by threshold value decisions in steps 2 and 3. In processstep 2, window-integrated acceleration signal Δa is compared to a firstlower threshold S11 and, if the level of Δa drops below this thresholdS11, this indicates a sharper vehicle deceleration at the beginning ofthe crash. Signal Δa must also be compared to a second lower thresholdS12, which is even lower than first lower threshold S11. In the case ofa deployment crash, therefore, the vehicle decelerates so sharply thatsignal Δa drops below bottom threshold S12. In the case of anondeployment crash, signal Δa always drops below first lower thresholdS11, but not second lower threshold S12. In this manner, comparingsignal Δa to both lower thresholds S11 and S12 makes it possible todistinguish between nondeployment and deployment crashes. Ifwindow-integrated acceleration signal Δa exceeds an upper threshold S2,as indicated in process step 3, a typical elastic reverberation of thevehicle body occurs. If the AND function in process step 4 of the twothreshold value decisions yields a deceleration peak at the beginning ofthe acceleration signal, followed by an acceleration peak, the filterintervention described below can be carried out in process step 5. In adeviation from the method described, the analysis of acceleration signala to determine a nondeployment crash can also be carried out with othermethods, such as a frequency analysis.

Filter FI mentioned above subjects measured acceleration signal a to alow-pass filtration. Deployment threshold Δvs is derived from filteredacceleration signal af in diagram block SW, as described above. Thecut-off frequency of low-pass filter FI is set to yield a filter signalaf for deployment crashes, with a deployment threshold Δvs, resulting ina reliable deployment decision, being formed from this filter signal indiagram block SW. However, if diagram block NAE then detects anondeployment crash, the sensitivity of low-pass filter FI, i.e., thecut-off frequency of this filter is reduced for a specific time segment,so that resulting output signal af of filter FI produces a modifieddeployment characteristic in diagram block SW, which reliably preventsthe means of restraint from deploying.

FIG. 7a shows an example of a variation in a measured acceleration a.Beneath this figure, FIG. 7b illustrates low-pass-filtered variation afin measured acceleration signal a. The solid line is low-pass-filteredacceleration signal af at an unchanged cut-off frequency. If anondeployment crash is then detected at time t1, this reduces thecut-off frequency of low-pass filter FI, which is reflected in thevariation in filter output signal af represented by the broken line. Thecut-off frequency shift is maintained until a time t2, which occursroughly 15 ms after t1. The cut-off frequency of low-pass filter FI isthen returned to its original level so that any deployment crashes thatoccur thereafter can be reliably detected.

Low-pass filter FI is preferably an IIR (infinite impulse response)filter or an FIR (finite impulse response) filter whose filtercoefficients are varied accordingly to vary the cut-off frequency in anondeployment crash.

What is claim is:
 1. A method for forming a deployment criterion for arestrainer in a vehicle, comprising the steps of: measuring anacceleration signal of the vehicle in at least one direction; analyzingthe acceleration signal, upon measuring the acceleration signal, in afirst time segment; determining a nondeployment event based uponcharacteristic features indicating a crash event in which the restrainershould not be deployed; reducing a cut-off frequency of a low-passfilter for a predetermined time segment if a nondeployment event isdetermined; filtering the acceleration signal in the low-pass filter,yielding a filtered acceleration signal; integrating the accelerationsignal, yielding an integrated acceleration; and comparing theintegrated acceleration to a deployment threshold, the deploymentthreshold having a higher value at low accelerations than at highaccelerations, the deployment threshold being formed as a function ofthe filtered acceleration signal.
 2. The method according to claim 1wherein the step of integrating the acceleration signal does not beginuntil the acceleration signal exceeds a predetermined level.
 3. Themethod according to claim, 1 wherein the low-pass filter is an infiniteimpulse response filter whose parameters are varied upon determining anondeployment event so that the filter cut-off frequency is reduced. 4.The method according to claims 1, wherein the low-pass filter is afinite response filter whose parameters are varied upon detecting anondeployment event so that the filter cut-off frequency is reduced. 5.The method according to claim 1, further comprising the steps of:integrating the acceleration signal across multiple consecutive, shorttime windows in the first time segment to detect nondeployment events;subjecting the integrated signal to a threshold value decision; anddetermining a nondeployment event if the integrated signal initiallydrops below a lower threshold due to a vehicle deceleration andsubsequently exceeds an upper threshold due to elastic vibrations in abody of the vehicle.
 6. The method according to claim 5, furthercomprising the steps of: comparing the integrated signal to a furtherthreshold lower than the lower threshold, the signal only dropping belowthe further threshold in the case of deployment crashes; and determininga nondeployment event only if the integrated signal does not drop belowthe further threshold.